Normally, the braking equipment of a motor vehicle or parts of the braking equipment is or are also operated when the motor vehicle is stationary in order to keep the latter stationary. For this, use is generally made of a parking brake device, which is typically actuated by a vehicle driver, or of braking equipment which produces in a controlled manner braking forces required to keep the vehicle stationary.
To drive away the motor vehicle, it is necessary to reduce the braking forces produced when the vehicle is stationary.
In the case of conventional braking equipment to be operated by a vehicle driver, it is necessary for the vehicle driver, in addition to the measures required for driving away, also to take measures to release the brake (parking brake).
In order to assist a vehicle driver when driving away a motor vehicle, it is known from German Laid-Open Application 24 20 252 to deactivate the parking brake of a motor vehicle in dependence on a position of an accelerator pedal. In this case, the parking brake, which produces a fixedly preset braking force when the vehicle is stationary, is deactivated in order to reduce the fixedly preset braking force as soon as the accelerator pedal is actuated.
Driveaway assistance for a motor vehicle on an ascending slope is known from DE 36 18 532 A1, and corresponding U.S. Pat. No. 4,717,207, both of which are incorporated by reference herein. In this case, a braking system has a valve which is arranged between the master cylinder and the wheel-brake cylinders and which is closed when the vehicle is stationary in order to maintain a brake actuation pressure and opened in order to release the brake actuation pressure and allow the vehicle to be driven away. When the motor vehicle is to be brought from stationary to a driving state, it is checked whether the engine torque preset by a vehicle driver corresponds to a driveaway torque required for driving away. In this case, the required driveaway torque is determined in dependence on a current angle of inclination of the vehicle on an ascending slope and the vehicle weight.
Alternatively, provision is made in the case of this driveaway assistance to detect torques acting on driven wheels of the motor vehicle. If torques acting on the driven wheels are large enough to overcome the braking torques produced by the wheel-brake cylinders and the rolling-back torque brought about by an ascending slope, the valve is opened to release the brake actuation pressure produced when the vehicle is stationary.
A fundamental problem of these known systems is that the function for the driveaway assistance is always activated although it is not desired in all driving situations. For example, when driving away on an ascending or descending slope with a trailer attached or when parking on an ascending or descending slope, undesired driving states may arise owing to the automatic reduction of the braking pressure.